ls should use file names, not abs paths

src/builtin/ls.rs

@@ -11,8 +11,13 @@ impl Call for Ls {
         let cwd = env::current_dir()?;
         let dir = fs::read_dir(&cwd)?;
         for child in dir {
-            let child = child?;
+            let child_path = child?.path();
-            _ = write!(ctx.stdout, "{}\n", child.path().display());
+            let fname = child_path
+                .file_name()
+                .ok_or_else(|| ExecError::new("no file name"))?
+                .to_str()
+                .ok_or_else(|| ExecError::new("non-unicode file name"))?;
+            _ = write!(ctx.stdout, "{}\n", fname);
         }
         Ok(Value::None)
     }

src/error.rs

@@ -126,6 +126,10 @@ impl ExecError {
     pub fn type_error<S: Into<String>>(msg: S) -> Self {
         Self::ValueTypeError(msg.into())
     }
+
+    pub fn new<S: Into<String>>(msg: S) -> Self {
+        Self::Misc(msg.into())
+    }
 }
 
 impl Error {